1. Field of the Invention
This invention relates to an electric lead device for superconducting electric apparatus, and more specifically, to such an electric lead device adapted to connect a superconducting electric apparatus disposed in cryogenic equipment to the outside.
2. Description of the Prior Art
There has been known a conventional electric lead device for superconducting electric apparatus which is illustrated in FIGS. 5 and 6, and which is disclosed, for example, in Japanese Utility Model Application Laid Open No. 52-144676. In FIG. 5, a vertical section of a portion of the conventional electric lead device is shown in which the upper portion is at a normal temperature side and the lower portion is at a cryogenic temperature side. The conventional lead device illustrated includes a plurality of conductors 1 received in a lead housing 2 formed of a metal pipe which is provided on its inner surface with an insulation layer 3 for effecting electric insulation between the conductors 1 and the lead housing 2. The conductors 1 are supported by electrically conductive discs 4 secured on their annular outer peripheries to the inner surface of the lead housing 2 through the insulation layer 3. Each of the electrically conductive discs 4 has a plurality of first small holes 5 formed therethrough into which the conductors 1 are inserted so as to be supported by the discs 4, and a plurality of second small holes 6 formed therethrough for passage of a coolant gas such as helium in the lead housing 2, the coolant gas being vaporized from a cryogenic coolant stored in a coolant tank (not shown) disposed below the lead housing 2.
With the above-described construction, coolant gas such as low temperature helium vaporized from the cryogenic coolant stored at cryogenic temperatures in the coolant tank (not shown) located below the lead housing 2 passes upwards through the second holes 6 in the discs 4 and removes electric resistance heat created in the conductors 1 as well as conduction heat conducted from the normal temperature portion located above the lead housing 2. In this connection, due to the fact that the lower the temperature of the conductors 1 is, the smaller the electric resistance thereof becomes, the conductors 1 are constructed so that the total cross sectional area of the conductors 1 decreases from the normal temperature side (the upper end in FIG. 5) toward the cryogenic temperature side (the lower end in FIG. 5) thereof so as to reduce heat conduction from the normal temperature side toward the cryogenic temperature side through the conductors 1 to thereby minimize the total amount of heat transmission of electric resistance heat and conduction heat through the conductors 1.
With a conventional electric lead device as constructed above, the greatest cross sectional area of the conductors 1 at their higher temperature side is limited particularly in case of a lead device for a small current rating, and hence it is difficult or substantially impossible to further reduce the cross sectional area of the conductors 1 at their cryogenic temperature side. Moreover, the cooling efficiency of the vaporized coolant gas such as helium in the lead housing 2 relative to the conductors 1 is determined by the cross sectional area of the conductors 1 so that reduction in cross sectional area of the conductors 1 results in decreased cooling efficiency.